A typical traction elevator includes a car, a counterweight, and a plurality of ropes interconnecting the car and counterweight. The ropes are engaged with a traction sheave that is rotated by a machine. Rotation of the traction sheave moves the car and counterweight through the hoistway.
In order to prevent the car from over-speeding in the downward direction, the car includes a safety device that is triggered by a governor. In a conventional configuration, if the car begins to travel faster than a predetermirned speed, the governor will cause a lift rod to be lifted relative to the car. As the lift rods are lifted, a pair of wedge shaped blocks are pulled into contact with the guide rail for the car. Friction between the guide rail and the wedge shaped blocks pulls the blocks into further engagement with the guide rails and thereby results in the application of a braking force to bring the car to a safe stop.
Such safety devices have proven reliable through use. A drawback to such devices, however, is that they work in only one direction, typically the down direction for the car. It is now desirable to prevent an over-speed condition from occurring in the up direction of the car as well. A simple solution to this problem is to install safety brakes on both the car and the counterweight. This solution may be expensive due to the doubling of the number of safety brakes.
Another possible solution is to place a spring-loaded brake on the car that stops the movement of the car in either the up or down direction. Due to the loads involved, this requires significant braking force between the braking surface and the guide rail and therefore the spring must be large enough to provide an adequate amount of force. A more significant problem is that during normal operation, the braking force of the safety brake must be removed to permit the car to travel through the hoistway. The removal of the braking force requires a device that applies an opposing force to overcome the force applied by the spring. In addition, the means to apply an opposing force should permit the spring to apply a braking force in the event of a loss of power. A possible solution is to provide an electromagnetic device, such as a solenoid, that counters the braking force when power is applied to the coil of the solenoid. Unfortunately, the forces required result in a large, heavy solenoid and a require a significant amount of power. Therefore, such a configuration is not practical.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop improved braking systems for elevators.